The present invention relates to an automated automotive test system. In particular, the present invention is an automated breakout box for performing test sequences on electrical systems of an automotive vehicle which include voltage and resistance measurements.
With the advent of integrated circuits, and with the increasing use of sophisticated electronic, electric and electromechanical sensors and control systems in automobiles, the testing and servicing of automobiles has presented ever-increasing challenges. At the present time, many tests of electrical systems, sensors and components of an automobile are made manually by a technician who unplugs a multipin connector between the vehicle's electronic control module (ECM) and the vehicle sensors and controls. The technician makes voltage and resistance measurements between various pins of the connector using a multimeter. The technician making those measurements is required to move the multimeter probe to appropriate measurement points. It is also necessary to set the multimeter to the proper measuring formula, such as volts or ohms.
This manual testing procedure has many drawbacks. In particular, the technician must follow detailed instructions in order to perform the proper tests. The instructions require that the technician set the proper condition, select the proper test points, read the equipment properly, and compare the reading to the listed tolerances specified in the instructions.
Some test systems have been developed which simplify some of the test operations. These devices use selection switches to select the proper test points. While this helps to some extent, the technician is still required to use the instructions, set the multimeter, and compare readings to the listed tolerances for that measurement.
The Ballou et al. U.S. Pat. No. 4,757,463 describes a computerized automotive vehicle diagnostic system in which a multiconductor probe cable is connected between the ECM and the vehicle sensors and controls. This cable is connected to a programmable cross-point switch, which is controlled by a microcontroller. The cross-point switch is controlled to connect a multimeter, a voltage source and a ground selectively to points in order to perform a sequence of tests. These can include reading resistance, reading voltage, reading a pulse width modulated signal and reading the time in between two pulses.
As the number of electrical systems and sensors increase, the number of possible combinations of points which need testing increases as well. An automated breakout box, therefore, would preferably make use of solid state switching devices to make connections between the testing circuitry and the points to be tested. Solid state multiplexers are available which are capable of selecting among a large number of different inputs or outputs. A solid state multiplexer offers significant advantages in size, cost and reliability over a multiplexer which makes use of electromechanical relays.
The problem with solid state multiplexers, however, is the internal resistance of the multiplexers in the ON state--which can typically be 75 ohms, and which varies with temperature, humidity, and other factors. In comparison, the contact resistance in electromechanical relay is much less, and is not affected to the same extent by temperature or humidity.
In certain applications, the internal resistance of solid state multiplexers is not a factor. For example, in reading voltages, solid state multiplexers do not introduce a significant error, because the voltage measuring devices require high impedance input circuits (typically 10 megohms input impedance), and therefore the internal resistance of the multiplexers does not affect accuracy of a voltage reading. This is not the case, however, in measuring resistances of the type encountered in the typical automobile. Often, the internal resistance of a solid state multiplexer is much greater than the resistance to be measured. Measuring resistance of vehicle sensors and other control components requires the ability to make very precise resistance readings down to very low values (on the order of a few ohms). These readings must also be made at low current levels, so that the test current being used does not damage the circuitry or components during the testing procedure.
The use of relays in a multiplexer overcomes the problem of internal resistance, but has other disadvantages. For example, with a 60-channel multiplexer (i.e., which is required to make measurements between 60 pairs of contact points), a minimum of 120 relays is required. The resulting multiplexer is bulky, costly and not as reliable as solid state multiplexers.